1) Field of the Invention
The present invention relates to an optical fiber, a manufacturing method of the same, and an optical amplifier, and in particular, to a technique suitable to be used in a Wavelength Division Multiplex transmission system in which a wavelength interval between signals is decreased in comparison with a conventional technique in order to obtain a larger transmission capacity.
2) Description of the Related Art
A Wavelength Division Multiplex (WDM) transmission system, in which signal light having different wavelengths is transmitted through a single optical fiber, has been put into practical use as means to realize a larger transmission capacity in an optical communications system. Examples of key devices of the WDM transmission system include an optical amplifier that compensates an optical loss at the fiber, and as an example of optical amplifiers that can handle signal light in a C band (1529-1563 nm), an Erbium Doped Fiber Amplifier (EDFA) in which Erbium Doped Fiber (EDF) is employed as an amplifying medium is used. Then, an L band EDFA that amplifies signal light in an L band (1570-1608 nm) that is on a longer wavelength side of the C band is developed, and the capacity of the optical transmission system is further increased by using this L band EDFA in combination with the conventional C band EDFA.
Conventionally, a typical EDF that is used as an amplifying medium includes, as shown in FIG. 20, a core portion 22 and a clad portion 23. A refractive index of the core portion 22 is greater than a refractive index of the clad portion 23, and therefore, most of the signal light and pump light propagate through a fiber by repeating a total reflection in the core portion 22. Further, Erbium ions (Er3+) that contribute to amplification of the signal light are added to an entirety of or a portion of the core portion 22, and energy that the Erbium ions have absorbed from the pump light is transferred to the signal light, thereby amplifying the signal light.
Here, the above L band EDFA has a characteristic that a gain per unit length of the EDF as an amplifying medium is small. Therefore, in order to obtain a sufficient gain as an amplifier, it is necessary to increase a length of the EDF. On the other hand, there has been a problem that an increased length of the EDF can results in a greater influence of a crosstalk due to Four Wave Mixing (FWM), which is a nonlinear effect occurring in the EDF, leading to deterioration of transmission performance.
In order to address the above problem, there have already been proposed several methods to improve transmission performance of an optical amplifier. Techniques disclosed in Patent Document 1 and Patent Document 2 are among such methods.
According to the technique disclosed in Patent Document 1, it is possible to suppress the nonlinear effect such as the FWM by providing a first core portion, a second core portion provided on a side of an outer circumference from the first core portion and having a refractive index smaller than that of the first core portion, and a clad portion provided on a side of an outer circumference from the second core portion and having a refractive index smaller than that of the second core portion, by doping the first core portion and the second core portion with at least one type of rare earth element, respectively, and by using a DSC (Dual Shape Cores fiber) type refractive index profile for a refractive index distribution.
Further, according to the technique disclosed in Patent Document 2, by using a fiber for optical amplification that is a silica based fiber including a first core portion, a second core portion surrounding the first core portion and having a refractive index lower than that of the first core portion wherein a difference between the refractive indexes is from 0.2 to 1.0%, and a clad portion surrounding the second core portion and having a refractive index lower than that of the second core portion wherein a difference between the refractive indexes is from 0.3 to 2.0%, in which the first core portion is a silica based glass which is a doped rare earth element, pump light can propagate for a long distance through the second core portion which is low loss because it is a silica based glass, and thus it is possible to increase gain efficiency of the amplifier.    Patent Document 1: Japanese Patent Laid-Open No. 2004-4772    Patent Document 2: Japanese Patent Laid-Open No. HEI 3-238883